Will my umbilical cord be clean? When should I stop?

April • 🎂 33, 👦🏼9/25/08, 👱🏼‍♀️02/16/14, 👼🏼👼🏼10/26/15, 👧🏼1/1/19, 👶🏼8/1/20, 🤰🏻edd 02/21/22,👫🏻💜4/21/17, 💉RN,⚡️Ravenclaw, 🔪👻Dean+Sam, 🔮 Tarot, 🪐Astrology ♒️♒️♒️, Energy 🧘🏻, Holistic Healing 🌿

I’ve been honest with my doctor and let him know I smoke occaasionally when I have insomnia, nausea, pain, etc during this pregnancy. When should I stop smoking to test negative in the umbilical cord at birth? I’m 28 weeks and tend to deliver early (36 weeks with my 1st, 34 weeks 6 days with my second but didn’t smoke With either of them)